1. Field of the Invention
The present invention relates to imaging apparatuses for picking up images using solid-state image pickup devices and image processors for performing color correction processing upon input image signals, and, more particularly, to an imaging apparatus having a linear matrix operation function and an image processor.
2. Description of the Related Art
A linear matrix operation technique is attracting attention as a technique for correctly reproducing colors in imaging apparatuses that use solid-state image pickup devices, such as a digital still camera and a digital video camera. The linear matrix operation technique improves color reproducibility by performing a linear matrix operation corresponding to the following equation 1 upon R, G, and B input signals to bring the spectral characteristic of each component of the R, G, and B input signals closer to human vision characteristics.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                      (                                                                      R                  ′                                                                                                      G                  ′                                                                                                      B                  ′                                                              )                =                              (                                                            a                                                  b                                                  c                                                                              d                                                  e                                                  f                                                                              g                                                  h                                                  i                                                      )                    ⁢                      (                                                            R                                                                              G                                                                              B                                                      )                                              (        1        )            
FIG. 9 is a block diagram showing the configuration of a known imaging apparatus that has a linear matrix operation function.
Referring to FIG. 9, light incident from an optical block 101 is photoelectrically converted into an analog image signal by an image pickup device 102. An A/D (analog/digital) conversion circuit 103 digitally converts the analog image signal transmitted from the image pickup device 102. A pre-correction circuit 104 performs upon the digital image signal various types of correction processing (pre-correction processing) related to the image pickup device 102 and an optical system, such as digital clamp processing for adjusting a black level, defect correction processing for correcting a signal output from a defective pixel of the image pickup device 102, and shading correction processing for correcting light falloff around a lens. A demosaic processing circuit 105 performs demosaic processing upon the signal output from the pre-correction circuit 104 so as to generate three R, G, and B plain signals (R, G, and B signals at the same spatial position) from R, G, and B signals with different spatial phases.
A linear matrix (LM) operation circuit 106 performs the above-described linear matrix operation upon the R, G, and B signals output from the demosaic processing circuit 105. Matrix coefficients used for the operation (computation) performed by the LM operation circuit 106 are set by a computation unit 107 configured with a microcontroller. The computation unit 107 sets, for the R, G, and B signals, the matrix coefficients that can bring the spectral characteristics of individual components of the image pickup device 102 closer to color matching functions approximately equal to human vision characteristics.
The R, G, and B signals output from the linear matrix operation circuit 106 are input into a white balance (WB) control circuit 108 and an integration circuit 109. The white balance control circuit 108 adjusts the gain of each component of the R, G, and B signal components. The integration circuit 109 detects the R, G, and B signals to be input into the white balance control circuit 108. The computation unit 107 controls the gain of the white balance control circuit 108 in accordance with R, G, and B integration values obtained by the integration circuit 109 so that the values of the R, G, and B signal components can become equal to each other for a white subject in an input image.
A gamma (γ) correction circuit 110 performs gamma correction upon the white balance controlled R, G, and B signals. The gamma corrected R, G, and B signals are input into a Y signal (brightness signal) processing circuit 111 and a C signal (color-difference signal) processing circuit 112, and undergo computations in each circuit, and are then separated into a Y signal, a Cr (R-Y) signal, and a Cb (B-Y) signal. The separated signals are output to a graphic processing circuit for generating an image to be displayed on a monitor, or a compression coding circuit for generating a signal to be recorded on a recording medium.
As a known imaging apparatus having such a linear matrix operation function, there is an imaging apparatus provided with a linear matrix circuit that can reduce the number of parameters and prevent circuit scale expansion by deriving, from two control parameters, six coefficients required for linear matrix conversion (see, for example, Japanese Unexamined Patent Application Publication No. 2000-50299 (paragraph Nos. 0013 to 0021 and FIG. 1)).
The linear matrix operation processing for improvement of color reproducibility causes a side effect in which noise is increased. The linear matrix operation processing is an effective method under ideal conditions of no noise. However, in a case where the linear matrix operation processing is performed so as to improve color reproducibility, the values of off-diagonal components (b, c, d, f, g, and h) among coefficients a through i shown in the above-described equation 1 sometimes become negative values. If subtraction is performed upon R, G, and B signals using such coefficients, a signal level (S component) is reduced, whereas the amount of noise (N component) is not reduced. Consequently, a signal-to-noise (S/N) ratio is undesirably reduced. In particular, since the signal level of an image is low under low illumination, image quality degradation due to a decrease in the S/N ratio is conspicuous.
A method of preventing a decrease in the S/N ratio under low illumination by setting coefficients for a linear matrix operation as a unit matrix can be considered. For example, in an example shown in FIG. 9, the computation unit 107 sets matrix coefficients as a unit matrix when a detected illumination level of image signals input into the linear matrix operation circuit 106 is low, and then sets the matrix coefficients for the linear matrix operation circuit 106. This control method is very effective, since, under low illumination, image quality degradation due to a decrease in the S/N ratio is more pronounced than color reproducibility deterioration. As will be described later, however, if matrix coefficients are changed under low illumination in a system shown in FIG. 9, the white balance of an output image is disturbed.
FIG. 10 is a timing chart showing the operation of each unit included in a known imaging apparatus when matrix coefficients are changed.
FIG. 10 shows exposure timing of the image pickup device 102, readout timing from the image pickup device 102, timing of various computations performed by the computation unit 107, and white balance control conditions each of which shows how the white balance of an output image based on a signal read out from the image pickup device 102 has been controlled. For example, it is assumed that, when the white balance of an output image is adequately controlled just before the timing T51, matrix coefficients for the linear matrix operation circuit 106 are changed by the computation unit 107 at timing T51.
Image quality control performed by changing coefficients affects an image signal output from the image pickup device 102 just after the coefficients have been changed, and R, G, and B integration values are detected from the image signal by the integration circuit 109 at timing T52. The computation unit 107 calculates a gain control value for a white balance on the basis of the detected R, G, and B integration values, and sets the calculated value for the white balance control circuit 108 at timing T53. Consequently, the white balance of an image signal output from the image pickup device 102 just after the timing T53 is adequately controlled.
If the above-described operations are performed, in a 2V-period between the timing T51 in which matrix coefficients are changed and the timing T53 in which a white balance gain is set, a white area used as a standard of reference of white balance control is shifted in the computation unit 107, whereby white-balance-disturbed images are output. Furthermore, for example, if the matrix coefficients are changed again at timing T54 after the white balance has been adequately controlled, white-balance-disturbed images are output again in a 2V-period between the timing T53 and timing T55.
FIG. 11 is a diagram showing exemplary set values of matrix coefficients corresponding to illumination levels.
shown in FIG. 11, if the matrix coefficients are continuously changed from an S/N-oriented setting to a color-reproducibility-oriented setting in the period between illumination levels L61 and L62, a white balance adequately controlled image and a white-balance-disturbed image are alternately output, that is, the color of each output image becomes unstable, in the transition period of coefficients between L61 and L62 similar to conditions shown in the period between T51 and T55 in FIG. 10. Such conditions cause significant image quality degradation, since color change due to white balance disturbance is relatively conspicuous.
As described with reference to FIGS. 9 through 11, if a linear matrix operation is performed so as to improve color reproducibility in the known imaging apparatus, the S/N ratio of an output image is decreased. If matrix coefficients are set as a unit matrix under low illumination so as to prevent the decrease in the S/N ratio, white balance control cannot follow suit every time the matrix coefficients are changed, whereby white-balance-disturbed images are output, and image quality is degraded.
It is desirable to provide an imaging apparatus capable of picking up a high-quality image with high color reproducibility and an adequate S/N ratio.
In addition, it is desirable to provide an image processor capable of outputting a high-quality image with high color reproducibility and an adequate S/N ratio.